This invention is directed to a method and apparatus for improving combustion and the operation of new or retrofitted boilers in various ways. The adoption of the proposed method and apparatus can be expected to reduce capital and operating costs. The types of boilers to which the invention applies are boilers burning biomass, wood waste or other solid fuel, and recovery boilers which burn waste liquor from various pulping processes which are employed in the manufacture of pulp and paper. These processes include: the kraft process, the soda process, the sodium-based sulphite process, the closed-cycle CTMP (chemical, thermal, mechanical pulp) process, the magnesium-based sulphite process and the ammonium-based sulphite process. These boilers generate steam for various process requirements.
All these boilers require combustion air and generally have furnaces which are rectangular in horizontal cross-section.
Ineffective combustion air systems result in poor mixing of the combustion air with the combustibles in the furnace. Poor mixing causes inefficient combustion, which can lead to excessive fouling of the heating surfaces, excessive erosion of the boiler tubes and overloading of the electrostatic precipitator. In certain recovery boilers, inefficient combustion also causes excessive emissions of TRS (total reduced sulphur), carbon monoxide and fume, unnecessarily low smelt-reduction efficiencies and may cause problems with char-bed control and smelt run-off. The manner in which the combustion air is introduced at the various elevations in the furnace may generate excessive fume.
Ineffective combustion air systems may also create a central column of rapidly-upward-flowing flue gases which entrains particulate and, in recovery boilers, liquor droplets and particulate, and carries this material out of the furnace. This carryover material can cause fouling of the heating surfaces and overloading of ash hoppers.
Ineffective combustion air systems may have air jets which fail to penetrate sufficiently far into the furnace, thus starving the centre of the furnace of oxygen. Alternatively, the air jets may be too strong and impinge on the opposite furnace wall, causing circulation problems and/or tube damage. Some combustion air systems suffer lack of jet penetration and/or excessive jet penetration when operated at loads other than the design load.
In boilers burning biomass, wood waste or other solid fuel, the fuel is generally burned on a grate, or in a fluidized bed. The combustion air is introduced both undergrate and through multiple air ports in the furnace walls. In this type of boiler, the air introduced through the wall ports is often termed overfire air, but the various air zones may be given the same terminology as the air zones in recovery boilers, as described below.
In recovery boilers firing liquor from the magnesium-based sulphite process, and the ammonium-based sulphite process, most of the combustion air is introduced as so-called primary air through liquor burners located in the walls or roof of the furnace while the remainder of the combustion air is introduced through multiple air ports in the furnace walls. These multiple air ports may be arranged in several zones, or sub-systems of ports, and may be named, successively, from the burner region towards the outlet of the furnace, secondary air and tertiary air, etc. The multiple ports of each air zone may be on one or more walls of the furnace.
In recovery boilers firing liquor from the kraft process, the soda process, the sodium-based sulphite process, and the closed-cycle CTMP process, all the combustion air is introduced through multiple air ports in the furnace walls. The air ports in these recovery boilers, and the air ports in the walls of boilers burning biomass, wood waste or other solid fuel, are arranged in several zones, or sub-systems of ports, named, successively, from the furnace floor elevation, upwards: primary air, secondary air and tertiary air, etc. The ports of each air zone may be on one or more walls of the furnace.
In recovery boilers which fire waste liquor from the kraft process, the soda process, the sodium-based sulphite process, and the closed-cycle CTMP process, conventionally, the primary air is introduced through multiple ports in four walls, such that the quantity of air originating from each wall is approximately the same and the flow through all the individual ports is more or less equal. The primary air jets from these ports on each wall collide with and interfere with the air jets from adjacent walls and are deflected upwards, thus creating the above-mentioned central column of rapidly-upward-flowing flue gases which causes particulate entrainment, fouling, etc. In a similar manner, the other air zones of the boiler can create or reinforce the central column of rapidly-upward-flowing flue gas, or create other regions of unnecessarily high upward velocities , which carry liquor droplets and other particulate out of the furnace.
By employing the proposed method and apparatus, upward velocity extremes are minimized, and gas mixing and combustion are improved. In certain cases, the number of air ports may be reduced. Thus, the operating and capital costs of the installation can be reduced.